The melanocortin-5 receptor (MC5R) is a G-protein coupled receptor (GPCR) belonging to the family of melanocortin receptors. There are five melanocortin receptors that have been isolated and cloned to date: MC1R, MC2R, MC3R, MC4R and MC5R. The melanocortin receptors participate in a variety of physiologic functions, providing a number of opportunities for therapeutic intervention in physiologic processes through alteration (i.e., a statistically significant increase or decrease) or modulation (e.g., up-regulation or down-regulation) of melanocortin receptor signalling activity.
Reviews of the melanocortin receptors, and their potential as targets for therapy have been published (Wikberg 2001; Bohm 2006). The melanocortin receptor family members are regulated by natural peptide agonists such as ACTH and the melanocyte-stimulating hormones (α-, β-, γ-MSH) derived from pro-opiomelanocortin (POMC) and by peptide antagonists such as Agouti signal protein (ASP) and Agouti-related peptide (AGRP). The MC1R is widely expressed and is associated with pigmentation in melanocytes and with inflammation responses in many cells involved in the immune system. The MC2R differs from the other melanocortin receptors in that it binds only ACTH but not MSH ligands. It is highly expressed in the adenal gland and controls corticosteroid synthesis. The MC3R is found in the brain, but also elsewhere in the body, and appears to play a role in the regulation of energy homeostasis, and possibly sexual dysfunction. The MC4R is found almost exclusively in the brain, with some reports of its presence elsewhere. It has been strongly associated with feeding control, and also implicated with sexual desire. The MC5R is widely expressed in peripheral tissues, particularly in the exocrine glands, with some receptor also expressed in the brain. Given the breadth of activity associated with the melanocortin receptors it is desirable when seeking to target one of these receptors to do so selectively in order to avoid side effects associated with antagonism or agonism of another receptor in this family.
The MC5R has been cloned and expressed from multiple species, including humans in 1993 (though called MC2 in this paper) (Chhajlani 1993), rat in 1994 (Griffon 1994) mice in 1994 (Gantz 1994; Labbe 1994) and in 1995 (Fathi 1995), canine (Houseknecht 2003), rhesus monkey (Huang 2000), sheep (Barrett 1994), zebrafish (Ringholm 2002), goldfish (Cerdá-Reverter 2003), spiny dogfish (Klovins 2004), rainbow trout (Haitina 2004), and chicken (Ling 2004), with the MC5R gene also identified in pig (Kim 2000). Patents covering the MC5R sequence in humans (Wikberg 2002), mice (Yamada 1997), rhesus monkey (Fong 2003) and dogs (Houseknecht 2003) have been published.
The MC5R has been implicated in regulating sebum secretion by a number of studies, as summarized in 2006 (Zhang 2006). Mice lacking MC5R have reduced sebum production, as evidenced by a marked inability to shed water from their fur, and a reduced quantity of sebum isolated from their hair. Significantly, these mice were otherwise generally healthy, with no readily visible abnormalities (appearance, behaviour, growth, muscle mass, adipose mass, reproduction, basal and stress-induced corticosterone, glucose and insulin levels) (Chen 1997). Further studies have identified reductions in pheromones, causing alterations in aggressive behaviours between mice (Caldwell 2002; Morgan 2004a; Morgan 2004b; Morgan 2006). Mice in which the POMC-derived peptide native ligands of MC5R have been knocked out show a similar phenotype (Yaswen 1999). Rats injected with α-MSH had 30-37% increased rates of sebum production, while removal of the neurointermediate lobe (the source of MSH) caused a 35% decrease in sebum secretion, which was restored upon administration of α-MSH (Thody 1973). A synergistic effect between α-MSH and testosterone was observed in rats, with testosterone increasing sebaceous gland and cell volumes (presumably via increased proliferation), α-MSH increasing dermal lipogenesis, and the combination increasing sebum secretion (Thody 1975a; Thody 1975b).
At a cellular level human sebocytes have been shown to express MC5R, via detection of MC5R transcripts in micro-dissected sebaceous glands (Thiboutot 2000), detection of MC5R in human facial sebaceous glands by immunostaining (Hatta 2001), detection of MC5R mRNA and MC5R in human sebaceous glands, cultured human sebocytes and rat preputial cells (Thiboutot 2000) and detection of MC5R as punctate particles within sebaceous glands by staining with polyclonal antibodies, seen in differentiated but not undifferentiated sebocytes (Zhang 2006). MC5R mRNA was also detected in sebaceous glands from the skin of wild-type mice, but not in skin sections of the MC5R-knockout mice (Chen 1997). Treatment of human sebocytes with cholera toxin (ChT), bovine pituitary extract (BPE), α-MSH or NDP-MSH increases lipid droplet formation, squalene synthesis, and MC5R expression (Zhang 2003; Zhang 2006), While both MC1R and MC5R have been detected in sebaceous cells, treatment of primary human sebocyte cell culture with NDP-MSH or BPE caused a substantial increase in human MC5R expression compared to serum-free conditions, correlating with sebocyte differentiation. Immortalized sebaceous cell lines (SZ-95, TSS-1 and SEB-1) also show MC5R expression (Jeong 2007; Smith 2007a; Phan 2007). These studies suggest that MC5R antagonists could be useful in reducing sebum secretion in mammals and hence in treating conditions associated with excess sebum secretion.
A family of 1,2,4-thiadiazole derivatives with MC5R antagonist activity (138-320 nM) were found to reduce sebum formation both in human sebocyte cell cultures and when applied topically to human skin grafted onto immunodeficient mice (Eisinger 2003a-d; 2006a,b).
Excessive sebum secretion, or seborrhoea, is a common affliction. Sebaceous glands occur over most of the body, with dense concentrations of large glands on the face, scalp and upper trunk (Simpson and Cunliffe p 43.1). Sebaceous secretion is dependent in part on androgenic hormones, possibly partly mediated by 5α-reductase processing of testosterone to 5α-DHT (dihydrotestosterone). Sebum consists of a species-specific mixture of lipids. In humans this consists of approximately 58% glycerides, 26% wax esters, 12% squalene, and 4% cholesterol/cholesterol esters (Simpson and Cunliffe p 43.5). The presence of squalene is almost exclusively characteristic of human sebum. The function of sebum is not well defined, but it is believed to have fungistatic properties, and play a role in moisture loss from, and water repellence of, the epidermis (Simpson and Cunliffe p 43.6; Danby 2005; Porter 2001; Shuster 1976; Kligman 1963).
Excessive sebum secretion has been associated with the development of acne vulgaris. Acne vulgaris is a common disease affecting an estimated 80% of the world's population at some stage in their lives. A person is more likely to develop acne than any other disease, although the severity varies greatly (Simpson and Cunliffe p 43.16). Acne peaks in prevalence and severity in adolescents aged 14-19 years old, with approximately 35-40% affected, but in a significant number of patients (7-24%) it persists beyond 25 years of age (Simpson and Cunliffe p 43.15). Of patients treated for acne, one study found 80% still had symptoms at 30-40 years of age (Simpson and Cunliffe p 43.16). While acne is not a life-threatening disease it can have a severe impact on a patient's quality of life (Follador 2006), with one study of severe acne patients showing similar impact as much more serious chronic medical conditions such as asthma, epilepsy, diabetes, back pain or arthritis (Mallon 1999).
Four major factors are believed to be involved in the pathogenesis of acne: (i) increased sebum production (seborrhoea), (ii) hypercornification/blockage of the pilosebaceous duct (comedogenesis), (iii) infection of the duct with P. acnes, and (iv) inflammation of the pilosebaceous duct (Simpson and Cunliffe p 43.15; Williams 2006). A number of studies have demonstrated a clear link between increased production of sebum, and the presence and severity of acne (Simpson and Cunliffe p 43.17; Youn 2005; Piérard 1987; Harris 1983; Cotterill 1981; Thody 1975c; Pochi 1964). A 2007 study found a correlation between sebum excretion and development of acne in preadolescent children (Mourelatos 2007). Sebum is the main nutrient of P. acnes, thus reduction of sebum will reduce the subsequent bacterial infection and inflammation response.
Androgenic sex hormones appear to play a role in the development of acne, with strong correlations with sebum production (Makrantonaki 2007). Two oral contraceptive pills are approved by the FDA for the treatment of acne vulgaris (Harper 2005), and these compounds appear to act by reducing androgen mediated sebum formation. Diet (Cordain 2005; Smith 2007b), stress (Zouboulis 2004) and genetic factors (Goulden 1999; Bataille 2006) also may play a role in acne, again potentially via increased sebum production.
Current treatments for acne vulgaris focus predominantly on treating the infection and inflammation stages of the disease, with a large number of different formulations of topical antibiotics (e.g. benzoyl peroxide, tetracycline, erythromycin, clindamycin) and retinoids (e.g. retinoic acid, isotretinoin, adapalene, tazarotene) in use, either alone or in combination; some of these also possess anti-inflammatory action (Simpson and Cunliffe p 43.36-43.38). Many of these treatments are of limited efficacy, particularly for severe cases of acne. A growing problem is the development of antibiotic-resistant strains of P. acnes (Simpson and Cunliffe p 43.37, 43.46; Williams 2006). Both topical retinoids and benzoyl peroxide cause skin irritation, and retinoids can cause photosensitivity (Williams 2006). Oral therapies include isotretinoin, antibiotics, hormones, and steroids. In females, antiandrogens have been shown to reduce sebum production (by approximately 40-80%, though with no placebo control group) and improve acne (Simpson and Cunliffe p 43.44; Burke 1984; Goodfellow 1984). Laser and UV-based therapies are gaining acceptance, and are believed to act through heating of the sebaceous gland followed by reduction in sebum formation; with reductions in both sebum formation and acne lesions measured (Jih 2006; Bhardwaj 2005). Of the many therapies available for acne, only oral isotretinoin and hormonal therapies act by regulating the sebaceous gland to reduce sebum secretion (Clarke 2007).
The most effective acne treatment, oral isotretinoin (13-cis-retinoic acid, Roaccutane, Accutane) was introduced in 1983 and still remains the most clinically effective anti-acne therapy. It is the only known treatment with strong sebusuppressive activity, reducing sebum excretion by up to 90% after 8-12 weeks of therapy (60-70% by 2 weeks) (Simpson and Cunliffe p 43.47; Jones 1983; Goldstein 1982; King 1982). Topical retinoids, in contrast, have no effect on sebum production. Oral isotretinoin is also anti-inflammatory, reduces comedogenesis, and reduces P. acnes infection. The mechanism of action is still unclear, and metabolites of isotretinoin appear to play a significant role. Isotretinoin induces apoptosis and cell cycle arrest in human immortalized SEB-1 sebocyte cell culture (Nelson 2006). Unfortunately, oral isotretinoin has serious side effects; most significantly it is a teratogen and requires a registration program for use in the USA. The FDA has issued a warning against online purchases of isotretinoin. Blood testing for fasting lipids and liver function is also recommended during treatment (Williams 2006). Isotretinoin has been implicated (though not substantively) with adverse psychological effects, including suicide and depression (Marqueling 2005).
Other forms of acne, such as acne conglobata or acne fulminans, may also respond to a sebum-reducing agent. Seborrhoea, or excessive skin oil production, is often associated with severe acne. Seborrheic dermatitis (SD) is a skin disease associated with sebum-rich areas of the scalp, face and trunk with scaly, flaky, itchy red skin affecting 3-5% of the population; dandruff represents a mild form of this dermatitis affecting 15-20% of the population. Seborrhoea and SD appear more common in patients with Parkinson's disease or mood disorders (facial paralysis, supraorbital injury, poliomyelitis, syringomyelia, quadriplegia, unilateral injury to the ganglion Gasser and those with HIV/AIDS) (Plewig 1999). Studies have shown that seborrheic dermatitis is also associated with chronic alcoholic pancreatitis, hepatitis C virus and various cancers. It is also common in patients with genetic disorders, such as Down's syndrome, Hailey-Hailey disease and cardio-facio cutaneous syndrome (Gupta 2004). MC5R antagonists may be useful for treating these indications.
Although rare, a variety of tumours involving sebaceous glands or sebaceous cells have been described (e.g. Ide 1999; Mariappan 2004; Kruse 2003). Muir-Torre syndrome consists of sebaceous gland adenomas associated with an internal adenocarcinoma (usually colon, breast, ovary or prostate). Preventing sebaceous cell differentiation may provide an effective treatment for arresting tumour growth. Oral isotretinoin has been used for this purpose (Graefe 2000). Sebaceous hyperplasia is a benign hyperplasia of the sebaceous glands, generating yellowish small papules on the skin surface, usually the face. The disease is associated with excessive undifferentiated sebocyte proliferation, but not excessive sebum formation. Ectopic sebaceous glands (Fordyce spots) are similar yellow papules found in the mouth or on the penile shaft. Both respond to oral isotreinoin. A compound that reduced sebocyte proliferation could be an effective treatment.
α-MSH shows immunosuppressive effects in humans, suppressing a variety of inflammation responses, and the MC5R has been implicated in these immunomodulating activities. MC5R mRNA was found to be expressed at high levels in human CD4+ T helper (Th) cells and in moderate levels in other human peripheral blood leukocytes (Andersen 2005). In mice, MC5R was detected in the lymphoid organs (Labbé, 1994), and MC5R was found on the surface of mouse pro-B-lymphocyte cells where it appears to mediate α-MSH activation of the JAK2 signalling pathway, enhancing cellular proliferation (Buggy 1998). Induction of CD25+ CD4+ regulatory T-cells by α-MSH also appears to be through MC5R (Taylor 2001).
For the reasons described above it would be desirable to provide MC5R antagonists that could be used in a number of therapeutic areas. Therapeutic regulation of biological signal transduction includes modulation of MC5R-mediated cellular events including, inter alia, inhibition or potentiation of interactions among MC5R-binding and activating or deactivating molecules, or of other agents that regulate MC5R activities. An increased ability to so regulate MC5R may facilitate the development of methods for modulating sebum secretion or other biological processes, and for treating conditions associated with such pathways such as acne as described above.
Accordingly there is still the need to develop improved methods of modulating the activity of MC5R which would facilitate the treatment of MC5R related conditions.